1. Field of the Invention
The present invention relates to a semiconductor device including a plurality of types of transistors different in required characteristics, and a method for manufacturing this semiconductor device, and in particular, it relates to a semiconductor device having a plurality of types of gate dielectric films different in film thickness and nitrogen concentration, and a method for efficiently manufacturing this semiconductor device.
2. Description of the Related Art
A few types of transistors are selectively produced according to required performance in a semiconductor device. When a gate dielectric film of a transistor is made thin, an on-current of the transistor increases, and high speed performance increases. However, when the gate dielectric film is thin, a tunnel current flows between a gate electrode and a substrate, a gate leak current increases, and a power consumption of the transistor increases. To the contrary, when the gate dielectric film is made thick, though the gate leak current decreases, the on-current decreases, and the high speed performance decreases. Thus, when the high speed performance is required for a transistor, its gate dielectric film is manufactured as thin. When it is necessary to reduce the power consumption of a transistor by restraining the gate leak current, its gate dielectric film is manufactured as thick. A silicon oxide or oxynitride film is generally used as the gate dielectric film.
A high performance transistor (an HP transistor) is used in a core unit of a conventional semiconductor device. The core unit is a part where a circuit for executing high speed arithmetic and logic processing is provided. The film thickness and a threshold voltage of the gate dielectric film of the HP transistor are set to lower than those of transistors provided in other parts. The HP transistor has a structure for giving priority to securing the on-current which determines the high speed capability of the transistor over restraining the gate leak current which increases as the film thickness of the gate dielectric film becomes lower, and restraining the off-current which increases as the threshold voltage decreases. The off-current is also referred to as a sub-threshold current in general, and is a leak current which flows between the source and the drain when the gate electric potential and the source electric potential is equal in a transistor, namely when the transistor is tuned off.
A transistor (an I/O transistor), whose gate withstand voltage is prioritized, is used in an I/O unit. The I/O unit is a part where a circuit for providing data for and receiving data from other semiconductor devices is provided. The film thickness of the gate dielectric film of the I/O transistor is set to be higher than that of the transistor in other parts, and its threshold voltage is set to be higher than that of the transistors in the core.
A lower power transistor (an LP transistor) is used in a low power unit. The low power unit is a part where a circuit whose leak current is restrained as low as possible is provided to control power consumption in a standby state. The film thickness of the gate dielectric film of the LP transistor is set to a value between the film thickness of the gate dielectric film in the core unit, and the film thickness of the gate dielectric film in the I/O unit. With this constitution, the gate leak current is restrained.
Further, a middle performance transistor (an MP transistor) whose characteristics are between those of the HP transistor and the LP transistor is formed on the same chip in some cases. Generally, the film thickness of the gate dielectric film of the MP transistor is set to equal to the film thickness of the gate dielectric film of the HP transistor. The off-current of the MP transistor is set to be lower than the one of the HP transistor by setting the threshold voltage thereof. The MP transistor is used in a core unit of a conventional semiconductor device.
As described above, a common gate dielectric film is generally used both for the HP transistor and the MP transistor. And, the film thickness of the gate dielectric film of the LP transistor is set to be higher than the film thickness of the gate dielectric film of the HP (MP) transistor, the film thickness of the gate dielectric film of the I/O transistor is set to be higher than the film thickness of the gate dielectric film of the LP transistor. Namely, three types of transistors, which are the core transistor (the HP transistor and the MP transistor), the LP transistor, and the I/O transistor, are used for a semiconductor device. The off-current of the LP transistor is about 1 to 50 pA/xcexcm, and the LP transistor is used for a circuit for which low power consumption is required. It is preferable to scale the gate dielectric film of the LP transistor, and to make it common with the gate dielectric film of the core transistor for simplifying a manufacturing step of the semiconductor device. However, when the gate dielectric films are made common, the gate leak current exceeds the off-current in a circuit where a low power consumption is prioritized, and the gate leak current determines the power consumption of the transistor. Because of the foregoing, the film thickness of the gate dielectric film of the LP transistor is not scaled, and is set to a film thickness different from that of the gate dielectric film of the core transistor (the HP transistor and the MP transistor). In this way, the film thicknesses of the gate dielectric films of the core transistor and the LP transistor are reduced almost to their limits in terms of the gate leak current.
A technique of introducing nitrogen (N) into the gate dielectric film that consists silicon oxide, and increasing the dielectric constant has been applied for simultaneously increasing the high speed performance and restraining the gate leak current of a transistor. Increasing the dielectric constant of the gate dielectric film allows decreasing an electrical film thickness of the gate dielectric film. As a result, the on-current of the transistor increases, and the speed of the transistor increases. Alternatively, the thickness of the gate dielectric film can be increased by an amount corresponding to the increase of the dielectric constant, and the gate leak current can be reduced.
As a method for introducing nitrogen into the gate dielectric film, heat treatment is applied to a silicon substrate in an NO atmosphere, for example. As another method, silane gas, O2 gas, and N2 gas are simultaneously supplied when the dielectric film is formed on the silicon substrate. Also, as another method, a silicon oxide film is annealed in an ammonia atmosphere. As yet another method, nitrogen is directly implanted into the silicon oxide film. However, the amount of nitrogen introduced into the silicon oxide film is about 2 to 3 atom %, and there is such a problem as the dielectric constant is not sufficiently increased in these methods.
Japanese Patent Publication Laid-Open No. Hei. 6-140392 discloses a method for radical-nitriding a silicon oxide film. With the method disclosed in Japanese Patent Publication Laid-Open No. Hei. 6-140392, a wafer on which a silicon oxide film is formed is loaded in a chamber, and is heated to 700 to 900xc2x0 C. Then, NH3 gas is introduced into the chamber, VUV plasma light emitting disc lamp is used to form Ar plasma, and nitrogen radical is generated. The generated nitrogen radical is used to directly nitride the silicon oxide film, and a silicon oxynitride film is formed. As a result, the silicon oxynitride film with a nitrogen concentration exceeding 10 atom % is formed. Nitrogen radical is nitrogen having one unpaired electron, and has larger energy and higher reactivity compared with non-radical nitrogen. Radical nitriding is also called as remote plasma nitriding.
However, the prior art has the following problems. Namely, when radical nitriding is simultaneously applied to a plurality of types of silicon oxide films having a film thickness different from one another, a larger amount of nitrogen is introduced into a silicon oxide film with a lower film thickness compared with a silicon oxide film with a higher film thickness.
Thus, in the thinnest silicon oxide film, the nitrogen reaches an interface between this silicon oxide film and the silicon substrate earliest. When a large amount of nitrogen reaches the interface between the silicon oxide film and the silicon substrate, a silicon nitride film is formed at this interface, and a physical film thickness of the dielectric film increases. When the film thickness increases excessively, the increase of the dielectric constant of the dielectric film does not compensate the increase of the film thickness, and the electrical film thickness increases as a result. Also, a large number of defects are generated at the interface, and the mobility of carriers decrease. As a result, the performance of the transistor decreases.
To increase the performance of the transistor, it is preferable to introduce as much nitrogen as possible into a thicker dielectric film, and to reduce an equivalent film thickness (the electrical film thickness). However, when excessive radical nitriding is applied in the conventional manufacturing method of a semiconductor, a large amount of nitrogen reaches the interface between the thinnest dielectric film and the semiconductor substrate, and the performance of a transistor decreases. In this way, when dielectric films with a different film thickness are simultaneously radical-nitrided in the prior art, nitrogen concentration introduced into the dielectric film with a lower film thickness increases more, and the nitriding may reach as far as the interface between the dielectric film and the semiconductor substrate.
An object of the present invention is to provide a semiconductor device which has a plurality of types of transistors whose gate dielectric films have a film thickness different from one another, and nitrogen concentration of these individual gate dielectric films is optimized. Another object of the present invention is to provide a method for manufacturing this semiconductor device.
A semiconductor device according to the present invention includes a semiconductor substrate, and a plurality of types of transistors provided with gate dielectric films different in film thickness and nitrogen concentration from one another. To form the gate dielectric films, a plurality of types of dielectric films different in film thickness and nitrogen concentration from one another are formed, and radical nitriding is applied to these dielectric films.
It is preferable that the plurality of types of gate dielectric films be formed such that the gate dielectric film with a higher film thickness has a higher nitrogen concentration. With this constitution, in the gate dielectric film with a lower film thickness, the nitrogen is prevented from reaching an interface between the gate dielectric film and the semiconductor substrate. Simultaneously, in the gate dielectric film with a higher film thickness, the nitrogen concentration is increased, the dielectric constant is increased, and the high speed performance is increased in a transistor provided with this gate dielectric film.
An alternative semiconductor device according to the present invention includes a semiconductor substrate, and a plurality of types of transistors provided with gate dielectric films different in film thickness and nitrogen concentration from one another. A non-radical nitrided layer is provided on the side in contact with the semiconductor substrate in the gate dielectric films. The non-radical nitrided layer means an area that the radical nitrogen does not reach in the radical nitriding.
In a method for manufacturing a semiconductor device according to the present invention, a plurality of types of dielectric films different in film thickness and nitrogen concentration from one another are formed on a semiconductor substrate. Then, radical nitriding is applied to these dielectric films to form a plurality of types of gate dielectric films.
In the present invention, since the dielectric films containing nitrogen are formed on the semiconductor substrate, the contained nitrogen blocks nitrogen introduced in the radical nitriding to arbitrarily control the amounts of the nitrogen introduced into the gate dielectric films by the radical nitriding. Therefore, by making the nitrogen concentrations in the plurality of types of dielectric films different from one another, the nitrogen concentrations in the plurality of types of gate dielectric films are individually and optimally controlled.
The plurality of types of dielectric films can be formed such that the dielectric film with a lower film thickness has a higher nitrogen concentration. Consequently, the introduction of the nitrogen is more effectively blocked in the dielectric film with a lower film thickness in the radical nitriding. As a result, in the dielectric film with a lower film thickness, the nitrogen is prevented from reaching an interface between the dielectric film and the semiconductor substrate. Simultaneously, in the dielectric film with a higher film thickness, a larger amount of nitrogen is introduced in the radical nitriding.
In the step of forming the plurality of types of dielectric films, it is possible to form a second and the following dielectric films as follows. That is, the surface of the semiconductor substrate is divided into a plurality of regions, and a first dielectric film is formed so as to cover the individual regions. Then, the first dielectric film formed on a second region is selectively removed, and a second dielectric film with a lower film thickness and a higher nitrogen concentration than the first dielectric film is formed on the second region. After the second dielectric film is formed, the dielectric film formed on an nth ((n) is a natural number equal to or more than 3) region is removed, and a dielectric film with a lower film thickness and a higher nitrogen concentration than a dielectric film formed on an (nxe2x88x921)th region is formed on the nth region. Consequently, since the second dielectric film whose film thickness is lower is formed after the formation of the first dielectric film whose film thickness is higher, the second dielectric film is not damaged when the first dielectric film is formed.
In the step of forming the plurality of types of dielectric films, it is also possible to form a second and the following dielectric films as follows. That is, the surface of the semiconductor substrate is divided into a plurality of regions, and a first dielectric film is formed so as to cover the individual regions. Then, the first dielectric film formed on a second region is selectively removed, and a second dielectric film with a higher film thickness and a lower nitrogen concentration than the first dielectric film is formed on the second region. After the second dielectric film is formed, the dielectric film formed on an nth ((n) is a natural number equal to 3 or more) region is removed, and a dielectric film with a higher film thickness and a lower nitrogen concentration than a dielectric film formed on an (nxe2x88x921)th region is formed on the nth region. A protection film may be formed on the first to (nxe2x88x921)th regions for preventing the dielectric film formed on the nth region from being formed on these regions when the dielectric film is formed on the nth region. With this constitution, since the second dielectric film is formed while the protection film is formed on the first dielectric film, it is possible to prevent the step for forming the second dielectric film from affecting the film thickness and the nitrogen concentration of the first dielectric film. The protection film may be a silicon nitride film, for example.
It is preferable that the semiconductor substrate be a silicon substrate, and the step of forming the dielectric film be a step of oxidizing or oxynitriding a surface layer of the silicon substrate to form a silicon oxide or oxynitride film. Consequently, since the surface layer of the silicon substrate is nitrided or oxynitrided, the dielectric films are easily formed.
It is preferable that in the radical nitriding, nitrogen radical be formed in a first chamber, the nitrogen radical be introduced into a second chamber, which is connected with the first chamber, and stores the semiconductor substrate, and the nitrogen radical come in contact with the dielectric film formed on the semiconductor substrate in the second chamber.
Consequently, plasma is formed to form the nitrogen radical outside the second chamber where the semiconductor substrate is placed. As a result, it is possible to prevent the plasma from damaging the dielectric film on the semiconductor substrate.
In an alternative method of manufacturing a semiconductor device according to the present invention, a plurality of types of dielectric films different in film thickness from one another are formed on a semiconductor substrate. Then, radical nitriding is applied to the dielectric films such that nitrogen does not reach an interface between the dielectric film with the lowest film thickness, and the semiconductor substrate. As a result, a plurality of types of gate dielectric films are formed.
In this way, with the present invention, when a semiconductor device includes a plurality of types of transistors, and the film thickness of the gate dielectric film of these transistors is different from one another, the nitrogen concentration is optimized in the individual gate dielectric films. Consequently, the characteristics of the individual transistors, namely the high speed performance and the gate leak current, are optimized.